Going from a reported 100% to a reported 90% after being removed from the charger can happen in just minutes or can take an hour or more (up to several in fact). It's not likely the battery being bad or falling to take a full charge. It's also not likely the phone using an inordinate amount of power over that short duration. It's a characteristic of how the charging and metering circuitry work.
When charging, the battery charger will charge the battery until it reaches what it dems a full charge, then shuts of charging completely. This is done for one very good reason...to protect the battery from overcharging and possible rupture. Once charging is interrupted, even while it's still connected to the charger, it's running on the battery, not the wall outlet. The charging and metering circuitry now beings monitoring the State of Charge (remaining charge level), and sits back while the battery is discharging. Once the battery reaches 90%, it resumes charging and replenishes the battery back to 100%, then repeats the cycle again.
When you pull it of the charger, you can be doing so when the battery has just completed a charge cycle and it's fully charged (100%), or it could be anywhere in the discharge process from 99% to 90%. Since the meter only displays the State of Charge in 10% increments over most of the capacity range, you can think you have 100% when you pull the plug, but you may only have 93% for example. As soon as you've used the 3%, the meter drops to 90%, leading you to believe the phone used 10% in a very short timeframe.
Another issue is when charging the phone while still powered on. If the phone is on, it's using power while charging. This causes the meter to become confused as to what the actual State of Charge is, sometimes causing it to interrupt the charging process prematurely. This can result in the phone reporting a 100% State of Charge when in fact it may be significantly less. This will result in the phone lasting somewhat less time during use than you should reasonably expect.
The only true way to assure a 100% charge is to charge with the phone powered off completely, and charging until the Charge-only meter displays 100%. If the phone is disconnected from charge and powered up within a short time from when the charge is complete, you are far more likely that it well remain at 100% for an appropriate timeframe, and that the phone will last the expected duration of the day during normal use.
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